The present invention relates to an electric heater of a gas detector of the type in which electrical resistance is varied when the electric heater contacts and burns a flammable gas or when the electric heater contacts and reacts with a gas such as carbon monoxide or chlorine, whereby the presence of a gas is detected in terms of the variation in electrical resistance.
In general, the contact-burn type gas detectors and semiconductor type gas detectors use an electric heater for heating a gas detecting element. Obviously, it is preferable that the eletric heater consumes less power and has a low thermal time constant. To this end, the electric heater must be designed and constructed very compactly in size so that its thermal capacity can be decreased to a minimum degree.
In these gas detectors, bridge circuits are almost exclusively used in order to measure the variation in electrical resistance across a gas detecting element. However, the changes of environmental conditions such as temperature change cause the unbalance of a bridge circuit, thus causing malfunctions. In order to overcome this problem, there has been proposed a method for inserting into a bridge circuit a compensating resistor having the same characteristics as a detecting element. In the gas detectors of the type described above, the characteristics of electric heaters must be uniform, but it is extremely difficult to design and construct the electric heaters with the uniform characteristics. In addition, the manufacturing costs are very expensive. Furthermore, it is also very difficult to design and construct in such a way that two electric heaters are disposed in relatively closely spaced apart relationship.
In the gas detectors of the type described above, a heating element consists of a film of an electrically conductive element deposited over a bridge of a silicon dioxide film as will be described in detail below. When the bridge and the conductive film over it are the same in width, the bridge cannot be heated uniformly over its entire length because heat is more easily dissipated at the ends than the center of the bridge. As a result, the center of the bridge is overheated and subsequently broken.
When the electric heater of the type described above is used as a temperature detector which detects the temperature in terms of the variations in electrical resistance in response to temperature changes of a substance used as the electric heater, the sensitivity of the temperature sensor becomes very low because such local heating as described above will not cause appreciable variations in electrical resistance of the electric heater.
The electric heater of the type described must have a small thermal time constant, a higher degree of response and a higher degree of sensitivity as well.